riscv: Expand the DT size before copy reserved memory node

The FDT blob might not have sufficient space to hold a copy of
reserved memory node. Expand it before the copy.

Reported-by: Rick Chen <rick@andestech.com>
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Rick Chen <rick@andestech.com>